Radial Velocity: Structures & Vocabulary

The radial-velocity method detects the line-of-sight motion of a star as it orbits the common center of mass with an unseen planet. Periodic Doppler shifts in stellar spectral lines reveal the orbital period and a lower limit on the planet's mass.

Star and planet both orbit their barycenter. As the star moves toward Earth, spectral lines shift slightly toward shorter wavelengths; as it moves away they shift longer. High-resolution stabilized spectrographs measure shifts corresponding to speeds of meters per second or less.

The discovery of 51 Pegasi b and many subsequent planets came from periodic stellar Doppler signals. Independent transit detections and astrometry confirm that these signals correspond to orbiting planets in many systems.

A velocity curve has period, phase, semi-amplitude, eccentricity, and argument-of-periastron information. Unknown inclination usually leaves a minimum mass m sin i.